How to insert a reference picture in 2d animation
You need to switch form Draw Mode to Object Mode



Then select the camera in the outliner.

In the properties window's settings for the camera you can add a "Camera Background Image". 



Once the image is loaded you can use it as reference in the background.

(or you can also choose to add an Image Reference object or an Image Background object in the 3d Viewport, but those images will not be tied to the camera.)

To go back to Draw mode, select the Stroke object in the Outliner and go back to Draw mode in the 3D Viewport.

Keep in mind that Camera Background and reference images will not be rendered.